The present invention relates to a gas sensor for measuring the concentration of a combustible gas component. More particularly, the invention relates to a gas sensor for measuring the concentration of a combustible gas component contained in exhaust gas from an internal combustion engine, and to an apparatus for measuring combustible gas component concentration by use of the gas sensor.
Japanese Patent Application Laid-Open (kokai) No. 8-247995 discloses an apparatus for measuring combustible gas component concentration having the following structure. In this apparatus, a gas under measurement which contains combustible gas components is introduced into a first processing zone. By means of a first oxygen pump cell, the partial pressure of oxygen is reduced to a predetermined low value (10.sup.-14 atm or less) at which combustible gas components remain substantially unburned. The thus-controlled atmosphere is introduced into a second processing zone. The combustible gas components contained in the atmosphere are burned with O.sub.2 which is pumped into the second processing zone by means of a second oxygen pump cell. At this time, pump current flowing through the second oxygen pump cell or voltage developed between electrodes of the second oxygen pump cell is detected. The combustible gas component concentration of the gas under measurement is determined from the detected value.
In the above-described apparatus for measuring combustible gas component concentration proposed in Japanese Patent Application Laid-Open (kokai) No. 8-247995, when O.sub.2 remains in the second processing zone, combustible gas components first burn through reaction with the residual O.sub.2. Accordingly, upon variation in the amount of O.sub.2 remaining in the second processing zone, no proportional relationship is established between the amount of O.sub.2 which is pumped into the second processing zone by means of the second oxygen pump cell and the amount of combustible gas components contained in the second processing zone. That is, this measuring apparatus suffers a measurement error, since, upon variation in the amount of O.sub.2 remaining in the second processing zone, no proportional relationship is established between current flowing through the second oxygen pump cell and the combustible gas component concentration of the second processing zone.
To avoid the above-mentioned problem, the partial pressure of oxygen contained in gas introduced into the second processing zone from the first processing zone must be reduced to a significantly low, constant level (10.sup.-14 atm or less). To fulfill this requirement, a driving force applied to the first oxygen pump cell must be increased, raising a new problem in that a burden imposed on the first oxygen pump cell increases. Additionally, if the partial pressure of oxygen is reduced excessively, dissociation of water will occur in the second processing zone, raising another problem in that a measurement error occurs.
Further, in this measuring apparatus, the operation of the second oxygen pump cell must be controlled such that the partial pressure of oxygen in the second processing zone becomes constant. To fulfill this requirement, in addition to the second oxygen pump cell, another element (e.g., oxygen concentration cell element) for measuring the partial pressure of oxygen in the second processing zone must be provided. Accordingly, the number of elements provided on the second processing zone side increases, causing complication of the sensor structure with a resultant increase in fabrication cost. Also, since the second oxygen pump cell is operated after detection of the partial pressure of oxygen in the second processing zone, responsiveness becomes poor.